Conventionally, various devices are known as fluid pressure devices provided with pistons. For example, fluid pressure cylinders including pistons displaced by the effect of supplied pressurized fluid are well known as means (actuators) for transporting workpieces and the like. A typical fluid pressure cylinder includes a cylinder tube, a piston disposed inside the cylinder tube so as to be movable in the axial direction, and a piston rod connected to the piston (for example, see Japanese Laid-Open Patent Publication No. 2003-120602 below). A packing composed of an elastic member is mounted on an outer circumferential part of the piston. In such a fluid pressure cylinder, when pressurized fluid such as air is supplied into the cylinder tube, the piston is pushed by the pressurized fluid and displaced in the axial direction. This also causes the piston rod connected to the piston to be displaced in the axial direction.
Moreover, a conventional fluid pressure cylinder often includes a magnet installed on an outer circumferential part of a piston and a magnetic sensor attached to an outer surface of a cylinder tube in order to detect the working position of the piston (for example, see Japanese Laid-Open Patent Publication No. 2006-242341).